A typical application of ultrasonics involves exciting ultrasonic waves in a source using a transducer such as a piezoelectric transducer, transferring the ultrasonic waves from the source to a target medium, and then determining properties of a study object or the medium itself by measuring propagation characteristics of the ultrasonic waves.
One of the important problems in ultrasonics is how to transfer ultrasonic energy from sources of ultrasonic energy to fluid media such as air with a large bandwidth at frequencies in the MHz range. This problem is particularly difficult in air, due to its very low acoustic impedance (.about.400 Rayls) as compared to the common source impedances, which are on the order of 10.sup.6 Rayls for a piezoelectric transducer. Even in water, which has a much higher acoustic impedance (.about.1.5.times.10.sup.6) than air, several impedance matching layers between the source and the medium are required to achieve a reasonable transfer efficiency. Thus, there is a need for technology that provides for the efficient transfer of ultrasonic energy from high impedance sources to low impedance media without requiring a large number of matching layers.
Applications that require ultrasonic energy to be coupled into media of low acoustic impedance include medical imaging, nondestructive testing, process monitoring, pressure sensing, flow detection and robotic sensing. To achieve the necessary time and space resolution, these applications typically require ultrasonic signals in the MHz frequency range with high signal to noise ratio. Thus, there is a need for ultrasonic transducers that can efficiently couple high frequency ultrasonic waves into low impedance media with high signal to noise ratios.